Method and device for determining any fluid mixture composition and for measuring material quantity

ABSTRACT

A method for determining any fluid mixture composition and for measuring material quantity by detecting light absorption, especially infrared light, emitted by an emitter ( 12.1, 12.2 ) in a fluid to be analyzed having non absorbed and non reflected components that are detected by a receptor ( 18.1, 18.2 ). According to said method, light is emitted directly from the emitter ( 12.1, 12.2 ) to the receptor ( 18.1, 18.2 ) through said fluid. Additionally, the light going through a channel ( 10, 16 ) before and/or after the fluid is dispersed at least once. The distance between emitter ( 12.1, 12.2 ) and receptor ( 18.1, 18.2 ) can be continuously altered. Electronic components are provided with a switch enabling to remove any interference light.

The invention relates to a method for determining the mixturecomposition of a liquid by means of determining the absorption of light,in particular infrared light, which is emitted by a transmitter in ahousing into the medium to be examined and whose nonabsorbed ornonreflected components are registered by a receiver in a housing, thelight being sent through the medium directly from transmitter toreceiver.

In many sectors, in particular industrial sectors, it is necessary todetermine the mixture composition of a medium or the quantity of amaterial. Purely by way of example, reference should be made toturbidity measurement in water treatment, determining the mixturecomposition in the foodstuffs industry, chemical and pharmaceuticalindustry, paper industry, textile industry, for example the level ofdyeing of a textile, the beverage industry, mining, etc., and tomeasuring the quantity of a material, for example when determining thinsheet thicknesses.

At present, use is substantially made of two methods, the scatteredlight measuring method and the transmitted light measuring method. Inturbidity measurement (turbidimetry), for relatively low turbidities themeasurement of the scattered radiation power (scattered light method) isbetter suited. In the case of this nephelometry, light is radiated intothe medium to be examined and is scattered at particles containedtherein. The intensity of the scattered light, which leaves themeasuring cell at a specific angle (normally 90°), is measured by aphotodiode.

In the case of relatively high turbidities, on the other hand, thetransmitted light measuring method is better suited which measuresphotometrically the decrease in radiation power during passage throughthe medium.

The disadvantage with all the known measuring instruments on the marketis that it is not possible to measure both particularly weakly absorbingand particularly strongly adsorbing media. Because of the principle, forexample, highly absorbent media cannot be measured by the scatteredlight method, since the light even cannot penetrate into the medium. Onthe other hand, if the distance between transmitter and receiver is toogreat in the transmitted light measuring method, then in the case ofvery highly absorbent media, this likewise leads to failure of themeasuring principle (in the case of too high a distance, there is simplyno measured signal which can be used).

Furthermore, in many instruments, the transmitter normally comprisesonly a single transmitting diode. Because of the cross-sectional area ofthe measuring beam which is present in this case and much to small, evenextremely small inhomogeneities or granularities of the medium lead towide fluctuations in the measured signal. In addition, such a measuringinstrument is extremely sensitive to deposits on the transmitting orreceiving diode.

Furthermore, the tiny extent of the measuring beam and the highlyinhomogeneous radiation field make it virtually impossible to align thetransmitter and receiver in the case of a variable arrangement.Extremely small deviations from this alignment, for example caused byvibrations or impacts, would result in drastic measured signal changesand erroneous calibrations.

EP-A 0 029 537 discloses a method for the detection of particles in agas stream. In this case, the particles fly through a laser beam. Thechange in the intensity of light is registered by a photelectric sensor.

EP-A 0 463 166 discloses a device for measuring the optical density ofgas. In this case, a radiator, a measuring flow chamber and aphotoreceiver are accommodated along an optical axis. By means of thisdevice, the analysis of the optical density of motor vehicle exhaustgases is preferably carried out.

U.S. Pat. No. 4,687,337 concerns the determination of the coefficient ofan atmospheric aerosol. This is likewise done by determining the changein a light beam.

U.S. Pat. No. 5,572,032 describes an instrument for analyzing gas, withwhich two or more components can be determined simultaneously. Thecorresponding device comprises two measuring cells, light sources, gasfilter cells and so on.

A method of the abovementioned type is disclosed by EP-A 0 539 824. Thisshows a spectrometer for forming turbidimetric and calorimetricmeasurements. Provided in an appropriate housing is a light source whichsends light through appropriate optics and a filter. The light thenpasses through the sample to be determined. The light passing throughthe sample is collected by a detector. By contrast, a second detectorobserves a reference light beam, so that a comparison can be madebetween reference light beam and the light which has gone through thesample.

The present invention is based on the object of developing a method anda device of the aforementioned type with which both particularly weaklyabsorbing and particularly highly absorbing media can be measured and,in particular, stray light can almost completely be disregarded.

The features as claimed in claim 1 lead to the achievement of thisobject.

In a corresponding device, transmitter and/or receiver are inserted intothe interior of a housing, remote from a housing opening. This avoidsdirect stray light striking the receiver, in particular, and thereforedistorting the measured result. For this reason, transmitter and/orreceiver are also arranged in a preferably rod-like housing, thesepreferably rod-like housings being approximately parallel to each other,so that the housing openings of transmitter and receiver are locatedlaterally opposite each other. The medium can move freely between therods, but entry of direct stray light is ruled out to the greatestpossible extent.

In order to suppress stray light still further, it may prove to beexpedient to color the interior of the respective channels belonging totransmitter and receiver black. In this way, reflection of stray lightonto the receivers is avoided.

The distance between transmitter and receiver can be varied. In thepresent invention, this is done by the appropriate housings themselvesbeing displaced. The change in the distance can preferably be madecontinuously.

The ability to vary the distance is an essential point in the presentinvention. It makes it possible for a very close spacing to be selectedin the case of highly absorbent material, while the two housings arelocated further away from each other in the case of less absorbentmaterial.

The absorption coefficients of the respective media can fluctuate oversuch a large range that measuring instruments with a fixed spacingcannot be brought to cover a similarly large range of absorptioncoefficients, even with the most complicated electronics, as the presentinvention. The ability to vary the spacing continuously makes itpossible for the first time ever to adapt the measurement range of themeasuring instrument on site to the range of fluctuation of theabsorption coefficient of the respective medium. Otherwise, precisely inthe case of very highly absorbent media, several identical measuringinstruments, which differ merely in the spacing between transmitter andreceiver, in a grid of 0.5 mm, for example, would have to be tried outuntil the optimum spacing was found.

In the case of the housings, a metal, in particular stainless steel, ispreferably chosen for the housing material. This has the advantage thatit adapts very quickly to the temperature of the medium to be measuredand, as a result, measured results cannot be distorted by temperaturedifferences between the two preferably rod-like housings, more preciselythe electronics situated therein. Furthermore, these steel tubes,together with the other metal housing parts, form a coherent, virtuallycompletely closed Faraday cage, so that even extremely weak signals canbe processed without interference and crosstalk.

In a particularly preferred exemplary embodiment of the invention, asmall part of the light is branched off after the transmitter and fed toa dedicated circuit. This quantity of light is used as a comparativequantity with the quantity of light determined by the receivers afterthe passage of the light through the medium to be measured.

It has proven to be expedient to homogenize the light which passesthrough the medium and which, in the present invention, has a preferablylarge cross-sectional area. For this purpose, weakly or highlyscattering filter disks are provided at suitable points, which eliminatethe known hotspots (inhomogeneities in the radiation field) of infraredlight transmitters. Accordingly, the receivers are also arranged at acertain distance behind the scattering filter disks, in order to receivethis homogenized light to a suitable extent. The receivers for the mainquantity of light and also for the quantity branched off are preferablyreceiver diodes, these receiver diodes being arranged symmetricallybehind the respective filter disk. In this case, four receiver diodesarranged distributed have proven to be best.

Without eliminating the hotspots and without the large-area measuringfield preferably used in this invention, there would be highfluctuations in the measured signal in the case of granular or otherwiseinhomogeneous media.

In order to increase sensitivity of the entire device, it has proven tobe expedient to reduce the electric field lines between transmitter andreceiver to a minimum. According to the invention, this is done byarranging a metal grating between transmitter and receiver, the metalgrating letting through 80% to 90% of the light but only 1% to 0.1% ofthe electric field lines. This sensitivity is further improved by thelight passing through the metal grating being deflected in the channelin front of the receivers. This is done by means of an appropriatelyarranged deflection mirror.

In each case a circuit for suppressing stray light should be arrangeddownstream of the receivers. In a preferred exemplary embodiment, acurrent-voltage converter is in each case followed by a prefilter withfrequency-selective amplification and constant-light suppression, alock-in amplifier, a low-pass filter, an amplifier with interferencesignal limitation, the respective amplifiers with interference signallimitation then being connected to a common logarithmic amplifier. Thislogarithmic amplifier is followed by the output circuit, which has twooutputs, one for the more highly absorbent material component of amixture composition, the other for the more weakly absorbing materialcomponent.

The two outputs (0 . . . 10V) can be balanced quite simply for mostmedia. Balancing is carried out with two substance samples which have adifferent and known mixture composition. By rotating two settingpotentiometers, the desired output voltage is set.

By using this device according to the invention, far “more opaque”substances can be measured than hitherto.

Of course, the device according to the invention also meets therequirements of DIN EN 27027 and 150 7027.

Further advantages, features and details of the invention emerge fromthe following description of preferred exemplary embodiments and byusing the drawing; in the latter:

FIG. 1 shows a plan view of a device according to the invention fordetermining the solids and/or liquid content, in particular the watercontent, of a liquid medium;

FIG. 2 shows a partial and schematically illustrated detail of thedevice according to FIG. 1;

FIG. 3 shows a partial and schematically illustrated detail of a furtherexemplary embodiment of a device according to the invention similar tofigure;

FIG. 4 shows a block-diagram representation of the device and inparticular the circuit for the device according to FIG. 1.

A device according to the invention for determining the solids and/orliquid content, in particular the water content, of a liquid medium or,in more general terms, for determining the mixture composition of anydesired media or for measuring the quantity of a material has a basichousing 1 in which there is a circuit described further below. Seated onthe basic housing 1 are two preferably rod-like housings 2 and 3, whichare arranged at a distance d from each other. The medium to be examinedcan flow or move in an interspace 4 between the two housings 2 and 3.The distance d is otherwise to be adjustable continuously preferablybetween 0 and 50 mm.

According to FIG. 2, the housing 2 has a housing opening 5, into which afilter disk, preferably a weakly scattering disk 6, is inserted.Opposite this filter disk 6, in a further housing opening 7 of thehousing 3, there is a transparent disk 8, in particular a glass disk (orelse a quartz or sapphire disk).

In the interior of the housing 2, the filter disk 6 is adjoined by achannel 10, into the base 11 of which transmitters 12.1 and 12.2 arelet, preferably emitting an infrared light. This infrared light, whichis preferably generated by four symmetrically arranged transmitters,strikes a transparent mirror disk 13 before it passes through the filterdisk 6, a small part (about 10%) of the infrared light being deflectedinto a further channel 14, passing through a preferably highlyscattering filter disk 15 and being picked up by receivers 18.3 and18.4. These receivers 18.3 and 18.4 are preferably receiver diodes, foursuch receiver diodes preferably being arranged symmetrically behind thefilter disk 15.

Both the filter disk 6 and the filter disk 15 have the task of eveningout hotspots (asymmetries in the radiation field), which occur ininfrared transmitters, and in this way of homogenizing the entire beam.

Opposite the filter disk 6, the glass disk 8 is likewise followed by achannel 16, which is in turn closed off by a highly scattering filterdisk 17. Behind this filter disk 17 there are preferably four receivers(receiver diodes), only two receivers 18.1 and 18.2 being shown. Thisfilter disk 17 also has the purpose of evening out the radiation and oftransmitting it in a form evened out in this way to the four photoreceivers.

Incidentally, the housings 2 and 3 preferably consist of steel tubes,which achieves evening out of the heat, regardless of the medium withwhich the housings come into contact. The housings 2 and 3 reach thetemperature of the medium after an extremely short time. All thereceiver diodes 18.1, 18.2, 18.3 and 18.4 are therefore virtually alwaysat the same temperature.

The exemplary embodiment of a device according to the inventionaccording to FIG. 3 differs from that according to FIG. 2 in that ametal grating 34 is provided in the beam path between the transmitters12.1 and 12.2 and the receivers 18.1 and 18.2, and a deflection mirror35 is provided in the channel 16. The metal grating 34 is located in thedisk 8, but can also be arranged at any other desired point betweentransmitter around receiver. The metal grating has the advantage that itpermits about 80% to 90% of the light to pass through but only about 1%to 0.1% of the electric field. The electric field lines pass through thegrating clearances only with difficulty, and most electric field linesare attracted and picked up by the metal grating. By means of this metalgrating 34, the sensitivity of the device is increased quiteconsiderably, to be specific to an extent of about 1 to 10 million.

In order to increase this sensitivity once more, the deflection mirror35 is provided, which deflects the incoming beams for example through90° onto the receivers 18.1 and 18.2. This increase in the sensitivityof the device is of critical importance in particular in the vicinity,for example, of a submersible pump, from which a great deal ofinterference originates.

According to FIG. 4, the transmitters 12.1 and 12.2 arequartz-stabilized pulse generators for a frequency f. This frequency fis also input in each case into a lock-in amplifier 24 and 25, which arein each case connected into circuits following the receivers 18.3 and18.4 and 18.1 and 18.2.

Each receiver 18.3/18.4 and 18.1/18.2 is followed by a current-voltageconverter 19 and 20, respectively. The signals from this current-voltageconverter 19 and 20 then pass into a prefilter 21 and 22 withfrequency-selective amplification and constant-light suppression. Inthis prefilter, those signals whose frequency coincides with thefrequency f are particularly emphasized. All other signals are highlyattenuated.

An additional amplifier 23 with remotely controllable gain is preferablyalso arranged downstream of the prefilter 22 in the receiver part.

In both circuits, there next follow the abovementioned lock-inamplifiers 24 and 25, and the low-pass filters 26 and 27 required fortheir correct function. The functional principle of these subassembliesis sufficiently well known: only those signals whose frequency and phasecoincide with the pulse excitation frequency f are passed on. All othersignals are highly suppressed. The lock-in amplifiers 24 and 25 inconjunction with the filters 26 and 27 behave like extremely narrow-bandand extremely stable band pass filters. At the same time, the phaseselectivity leads to high suppression of undesired, capacitivelycross-coupled signal components.

The filters 26 and 27 are then in each case followed by an amplifier 28and 29 with interference signal limitation. Said amplifier has theobject of amplifying the measured signal once more and, in addition, ofintercepting the known instabilities with very small signals(log(0)=−∞!), known from the usual logarithmic amplifiers, and thenegative voltages, which are not permitted.

The measured signal which now results and is unaffected by interferenceand stray light is fed both from the amplifier 28 and from the amplifier29 to a logarithmic amplifier 30, which is followed by an output circuit31 which has two outputs 32 and 33. The solids content, or, in moregeneral terms, the more highly absorbent material component (0-10 volts)can be indicated at the output 32, and the water content or, in moregeneral terms, the more weakly absorbing material component of a mixturecomposition (0-10 volts) can be indicated at the output 33.

List of item numbers  1 basic housing  2 housing  3 housing  4interspace  5 housing opening  6 filter disk  7 housing opening  8 disk 9 10 channel 11 base 12 transmitter 13 mirror disk 14 channel 15 filterdisk 16 channel 17 filter disk 18 receiver 19 current-voltage converter20 current-voltage converter 21 prefilter 22 prefilter 23 additionalamplifier 24 lock-in amplifier 25 lock-in amplifier 26 low-pass filter27 low-pass filter 28 amplifier 29 amplifier 30 logarithmic amplifier 31output circuit 32 output 33 output 34 metal grating 35 deflection mirrord distance

1. A method for determining the mixture composition of any desired mediaor for measuring the quantity of a material by means of determining theabsorption of light, in particular infrared light, which is emitted by atransmitter (12.1, 12.2) into the medium to be examined and whosenonabsorbed and nonreflected constituents are registered by a receiver(18.1, 18.2), the light being sent through the medium directly fromtransmitter (12.1, 12.2) to receiver (18.1, 18.2), characterized in thatthe light crosses a channel (10, 16) before and/or after the medium andis scattered at least once, wherein a distance (d) between transmitter(12.1, 12.2) and receiver (18.1, 18.2) is varied.
 2. The method asclaimed in claim 1, characterized in that part of the light is branchedafter the transmitter (12.1, 12.2) and is used in a circuit.
 3. Themethod as claimed in claim 2, characterized in that two outputs (32, 33)are provided, so that the mixture composition is indicated.
 4. Themethod as claimed in claim 3, characterized in that the two outputs (32,33) are balanced by using two material samples which have a differentand known mixture composition.
 5. The device for determining the mixturecomposition of any desired media or for measuring the quantity of amaterial by means of determining the absorption of light, in particularinfrared light, which is emitted by a transmitter (12.1, 12.2) into themedium and whose nonabsorbed and nonreflected constituents areregistered by a receiver (18.1), 18.2) characterized in that transmitter(12.1, 12.2) and/or receiver (18.1, 18.2) is/are inserted into theinterior of a housing (2, 3), remote from a housing opening (5, 7),further including means for varying a distance (d) between transmitter(12.1, 12.2) and receiver (18.1, 18.2).
 6. The device as claimed inclaim 5, characterized in that at least one scattering filter disk (6,17) is inserted between transmitter (12.1, 12.2) and receiver (18.1,18.2).
 7. The device as claimed in claim 5, characterized in thattransmitter (12.1, 12.2) and/or receiver (18.1, 18.2) are each arrangedin a preferably rod-like housing (2, 3).
 8. The device as claimed inclaim 7, characterized in that two rod-like housings (2, 3) are arrangedapproximately parallel to one another and leave a distance (4) betweenthem free for the medium to be examined, the housing openings (5, 7) oftransmitter (12.1, 12.2) and receiver (18.1, 18.2) being approximatelyopposite each other.
 9. The device as claimed in claim 8, characterizedin that a channel (10, 16) is provided between transmitter (12.1, 12.2)and the housing opening (5) assigned to the latter and/or betweenreceiver (18.1, 18.2) and the housing opening (7) assigned to thelatter.
 10. The device as claimed in claim 9, characterized in that thereceiver (18.1, 18.2) provided is a plurality of diodes arrangeddistributed and having a following current-voltage converter (20). 11.The device as claimed in claim 10, characterized in that the diodes(18.1, 18.2) are arranged at a distance behind a scattering filter disk(17).
 12. The device as claimed in claim 11, characterized in that ametal grating is arranged between transmitter (12.1, 12.2) and receiver(18.1, 18.2).
 13. The device as claimed in claim 11, characterized inthat a deflection mirror is arranged in the channel (16).
 14. The deviceas claimed in claim 13, characterized in that a device (13) fordeflecting part of the light is arranged downstream of the transmitter(12.1, 12.2).
 15. A device as claimed in claim 14, characterized in thatsaid deflection device is a transparent mirror disk (13).
 16. The deviceas claimed in claim 15, characterized in that the device (13) fordeflecting part of the light directs this light onto receivers (18.3,18.4), in particular diodes, arranged distributed at a distance behind apreferably highly scattering filter disk (15).
 17. The device as claimedin claim 16, characterized in that the receivers (18.1, 18.2) areconnected to an output circuit (31) via a circuit which includesstray-light suppression.
 18. The device as claimed in claim 17,characterized in that the current-voltage converter (19) that determinesthe quantity of light branched off is likewise connected to an outputcircuit (31) via a circuit which includes stray light suppression. 19.The device as claimed in claim 18, characterized in that the circuit hasa prefilter (21, 22) with frequency-selective amplification andconstant-light suppression, an additional amplifier with remotelycontrollable gain (23), a lock-in amplifier (24, 25), a filter (26, 27),an amplifier (28, 29) with interfering signal limitation and aconnection to a logarithmic amplifier (30), which is connected upstreamof the output circuit (31).
 20. The device as claimed in claim 19,characterized in that the lock-in amplifier (24, 25) in each case has aconnection to the transmitter (12.1, 12.2).
 21. The device as claimed inclaim 20, characterized in that the output circuit (31) has two outputs(32, 33), one for the more highly absorbent material component and theother for the more weakly absorbent material component of a mixturecomposition.
 22. The device as claimed in claim 21, characterized inthat the circuit/s are accommodated in a housing (1) from which thepreferred housing rods (2, 3) project.
 23. The device as claimed inclaim 22, characterized in that the preferred housing rods (2, 3)consist of steel, preferably stainless steel.